1. Field of the Invention
The present invention relates to a superconducting magnet device, and particularly, to a superconducting magnet device of the type in which a pulsating magnetic field generating coil is disposed on the inner diameter side of a cylindrical static magnetic field generating superconducting coil and which is for use in, for example, a magnetic resonance imaging apparatus or a nuclear magnetic resonance analyzing apparatus.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of one example of a conventional superconducting magnet device. In this device, superconducting coils 1 for generating a static magnetic field, which are in a superconductive state at a temperature of liquid helium, are accommodated within a liquid helium tank 2 filled with liquid helium 3. Supply of the liquid helium 3 into the liquid helium tank 2 and wiring of the superconducting coils 1 is performed through a service port 4 of the liquid helium tank. On the outer side of the liquid helium tank 2 is provided a liquid nitrogen tank 5 with liquid nitrogen 7 contained therein to act as a heat shield. The liquid nitrogen 7 is supplied into the liquid nitrogen tank 5 through a service port 6. Thermal shielding end plates 8 in contact with the liquid nitrogen tank 5 and a thermal shielding inner cylinder 9 in contact with the end plates 8 are maintained at the temperature of liquid nitrogen. All of these components are accommodated in a thermal insulating vacuum tank 10. At the center of the vacuum tank 10 are disposed normal conducting coils 11 for generating a pulsating magnetic field and a spool 12. The superconducting coils 1 and the pulsating magnetic field generating coils 11 in combination generate an associated magnetic field space 13.
Next, the operation of the above-described conventional device will be described below. When cooled by the liquid helium 3, the superconducting coils 1 go into a superconductive state with zero electrical resistance, and generate a static magnetic field without any loss in conduction of a DC current therein.
Since the latent heat of the liquid helium 3 is small, even a small amount of heat which enters from outside causes a large amount of liquid helium to evaporate. In order to cope with this, the superconducting magnet device is insulated or shielded against heat by the provision of the vacuum tank 10, the liquid nitrogen tank 5, the thermal shield end plates 8 and the thermal shielding inner cylinder 9 which are in contact with the liquid nitrogen tank 5, and the end plates 8 and the inner cylinder 9 are maintained at the liquid nitrogen temperature. This prevents heat from getting into the device as much as possible and thus restricts evaporation of the liquid helium 3. The thermal shielding of the inner cylinder 9 is generally made of a metal plate having a high heat conductivity, such as copper or aluminum. A pulsating current is applied to the pulsating magnetic field generating coils 11 which is in a normal conductive state, so as to superimpose a desired pulsating magnetic filed on the magnetostatic field generated by the superconducting coils 1 in the associated magnetic field space 13. Conduction of the pulsating current in the coils 11 generates an induced current in the inner cylinder 9.
As will be clear from the illustration shown in FIG. 1, the inner cylinder 9 surrounds the pulsating magnetic field generating coils 11 in a cylindrical fashion. This arrangement generates a high mutual inductance between inner cylinder 9 and the coils 11. Furthermore, the inner cylinder 9 is cooled to the liquid nitrogen temperature at which the electrical resistance thereof is low. These cause a large amount of induced current to be generated in the inner cylinder 9. The value of the induced current and the current applied to the pulsating magnetic field generating coils 11 have a relationship expressed by an equation (1). The pulsating magnetic field actually generated is the pulsating magnetic field generated by the current in the pulsating magnetic field generating coils 11 and the magnetic field generated by this induced current superimposed thereon, that is, the magnetic field generated by the induced current is added to the pulsating magnetic field oppositely. ##EQU1## where L: Self-inductance of the thermal shielding inner cylinder
M: Mutual-inductance between the pulsating magnetic field generating coils and the inner cylinder PA1 R: Resistance of the inner cylinder PA1 i.sub.l : Current applied to the pulsating magnetic field generating coils PA1 1.sub.2 : Induced current in the inner cylinder
For example, in a case where a current having a trapezoidal waveform shown in FIG. 2 (A) is applied to the pulsating magnetic field generating coils 11, the current that flows in the inner cylinder 9 is attenuated due to the resistance R of the inner cylinder 9, as shown in FIG. 2 (B). Accordingly, a magnetic field generated in the space 13 by both the currents shown in FIG. 2 (A) and 2 (B) varies with time, as shown in FIG. 2 (C). Hence, if it is desired that a magnetic field having a fixed intensity during a period indicated by T in FIG. 2 (C) is generated in the space 13, a current arranged such that it compensates for variations in the induced current in the inner cylinder 9 (see FIG. 3 (B)) has to be applied to the pulsating magnetic field generating coils 11, as shown in FIG. 3 (A) to provide for a magnetic field having a fixed value (see FIG. 3 (C)). An induced current is also generated in an inner cylinder 10aof the vacuum tank 10 disposed between the inner cylinder 9 and the normal conducting coils 11. However, the vacuum tank 10 has a normal temperature and, hence, a high resistance. Accordingly, the value of the induced current generated in the cylinder 10a is small enough to be attenuated to a neglected value.
In the thus-arranged conventional superconducting magnet device, in order to generate a pulsating magnetic field having a fixed intensity for a predetermined period of time, a current arranged such that it compensates for attenuation of the induced current in the thermal shielding inner cylinder 9 has to be applied to the pulsating magnetic field generating coils being in the normal conductive state, requiring that an exciting power source (not shown) of the pulsating magnetic field generating coils be equipped with this adjusting function.